JP2004508268A - Method of forming a defect-free, crack-free epitaxial film on a mismatched substrate - Google Patents

Abstract

A method for forming a crack-free epitaxial film having a thickness greater than that achievable by continuous epitaxial crystal growth. This epitaxial film can be used for devices and as a substrate for further epitaxy or can be used as a separate substrate separate from the initial substrate material (401) (407). In this method, an initial layer (403) having many defects for absorbing stress generated by epitaxy and another layer having few defects for flattening a crystal growth region are used as necessary, and a high-quality epitaxial region is provided near the surface. Is provided.

Description

[0001]
(Technical field)
The present invention relates to heteroepitaxial deposition of good quality, epitaxial, crack-free, low defect density films on thermal and / or lattice mismatched substrates. In particular, the invention relates to the heteroepitaxial growth of thick layers of compound semiconductor material for later use as a substrate for further deposition. More specifically, the present invention relates to a thick film made of gallium nitride (GaN), other group III nitrides (AlN, InN) and alloys thereof for use as a substrate material for further forming a device structure. About the deposition of.
[0002]
(Background of the Invention)
Gallium nitride (GaN) is recognized as an industrial material with great potential. For example, GaN has been used in the manufacture of blue light emitting diodes, semiconductor lasers, and other optoelectronic devices, and in the fabrication of high temperature electronic devices.
[0003]
One of the major challenges in mass-producing GaN-based devices is the lack of a suitable natural GaN substrate or a flat, thick GaN layer. Since GaN cannot be found in nature and its theoretical melting temperature exceeds the dissociation temperature at a moderate pressure, it cannot be melted like silicon, gallium arsenide, sapphire and pulled out of the bowl. However, forming extremely crystalline thin layers of GaN and related alloys for use in electronic devices requires homoepitaxial deposition of these layers on the surface of existing GaN. Such high quality device layers cannot be directly heteroepitaxially grown for reasons outside the scope of the present invention.
[0004]
Techniques currently used to form good quality GaN layers and related layers include heteroepitaxial deposition of GaN device layers on suitable but non-ideal substrates. Today, materials for such substrates include (but are not limited to) sapphire, silicon, silicon carbide, gallium arsenide, and the like. In all existing heteroepitaxial substrates, good quality GaN deposition is difficult due to lattice and thermal mismatch. Lattice mismatch is caused by different spacing between atoms in different types of crystals. Thermal mismatch is caused by differences in the coefficient of thermal expansion (CTE) between the joined dissimilar materials as the temperature rises and falls.
[0005]
The most commonly used heteroepitaxial substrate for GaN deposition is sapphire (Al), which has both large thermal and lattice mismatches with GaN. ₂ O ₃ ). For reasons unrelated to the scope of the present invention, sapphire has otherwise excellent properties as a heterosubstrate. However, large lattice mismatches result in films that are particularly undesirable from a device fabrication standpoint, particularly with a very high density of defects that appear in the form of dislocations. As with other epitaxial crystal growth processes, it is necessary to grow a GaN buffer layer on the surface of sapphire prior to forming a good quality layer for the device. The buffer layer will vary depending on the device's tolerance for dislocations, the use of special growth techniques (eg, growth using a mask pattern, use of a low temperature buffer layer, etc.), and other factors. Typically, the thickness of this GaN buffer is several microns. However, when the density of defects mainly in the form of dislocations is high (−10 to ¹⁰ cm ^-2 ), Resulting in poor device quality. In addition, since the sapphire substrate is not conductive and has a low thermal conductivity, its heat dissipation is limited, further reducing the performance of the device and complicating the device process.
[0006]
An easy solution to this problem is to increase the thickness of the GaN buffer layer in anticipation that the dislocation density will decrease as the distance from the interface with the substrate increases. Thick GaN buffer layers are also useful in device design and process, as they improve electrical and thermal properties. These very thick GaN buffer layers are called “temporary substrates”. The difficulty in growing a sufficiently thick GaN buffer layer on sapphire and then serving as a temporary substrate for the growth of a good quality layer for the device is due to the effects of thermal mismatch. Normally, GaN is deposited on sapphire at temperatures between 1000 and 1100 ° C., but when the sample is cooled to room temperature, the difference in the rate of thermal expansion (shrinkage) creates a high level of stress at the interface between the two materials. Let it. Since sapphire has a higher coefficient of thermal expansion (CTE) than GaN, with cooling, GaN is placed in a compressed state and sapphire is placed in tension due to interface mismatch. To some extent, the amount of stress is directly related to the thickness of the deposited GaN, so the larger the film thickness, the greater the stress. Above about 10 microns, the level of stress exceeds the breaking point of GaN, causing cracking and spalling of the film. Cracking in this layer is even less desirable than high dislocation densities and must be avoided because of the risk of catastrophic propagation to the device layer during subsequent processing.
[0007]
Referring to the drawings, FIGS. 1 (a) and 1 (b) schematically show the prior art where the deposition of a thick layer of GaN on sapphire is required. In FIG. 1A, a thick (greater than 10 microns) GaN film 102 is deposited on a sapphire substrate 101 at a growth temperature in the range of 1000 to 1100 ° C. The actual deposition method is irrelevant to the present invention. At the above temperature, there is no thermal stress because nucleation of the GaN film occurs on the substrate. FIG. 1 (b) shows the effect of a large temperature change when cooling the sample to room temperature. In the figure, the sapphire substrate 101 is in a state of being subjected to a tensile stress at present, and is curved concavely with respect to the deposited film. If the stress is sufficiently large, cracks 103 may be generated in the substrate. The epitaxial GaN layer 102 is in a state of being subjected to a compressive stress, and in addition to cracking, may be peeled off from the substrate 101 or may deteriorate the interface with the substrate 101.
[0008]
FIG. 2 schematically illustrates a series of steps involved in a method of forming a thick layer on a conventional thermal and / or lattice mismatched substrate. In step 201, a prepared substrate is prepared. The prepared substrate may be, for example, flat sapphire that has been chemically cleaned before use. In step 202, process parameters and growth conditions for growing a thick, flat and high quality layer are set. Step 203 deposits a thick layer on the prepared substrate. Preferably, the thickness of this layer is in the range of 10 to 400 microns. In step 204, the sample is cooled to room temperature and taken out of the reactor as it is. The wafer bows due to residual stresses caused by thermal mismatch between the epitaxial layer and the substrate. This stress can also cause numerous cracks in the thick layer and substrate.
[0009]
Accordingly, techniques for high quality, epitaxial, crack-free, low defect density films of arbitrary thickness, deposited on thermal and / or lattice mismatched substrates, and methods of depositing these films, are provided. It has been demanded.
[0010]
(Summary of the Invention)
The present invention provides a method for growing a crack-free layer of any thickness, consisting of GaN, or related III-V compounds or alloys, on a heteroepitaxial substrate. Still another object of the present invention is to provide a method for growing a film having an upper surface having fewer defects than an interface between a GaN film and a heteroepitaxial substrate. Another object of the present invention is to provide a method for forming an independent substrate made of GaN, AlN, InN or an alloy thereof.
[0011]
Another object of the present invention is to form a first stress-free, defect-rich, first heteroepitaxial layer, followed by forming a second, highly crystalline, homoepitaxial layer on the first layer. Is to provide a way to
[0012]
It is another object of the present invention to form a first stress-free, defect-rich first heteroepitaxial layer, and then to deposit a second, highly crystalline, homoepitaxial layer on the first layer. An object of the present invention is to provide a method for making a chew.
[0013]
Another object of the present invention is to form a first layer 1) which is highly defective and relaxed, followed by depositing a second layer 2) on the first layer, wherein the layer 2) is The purpose of the present invention is to show a method for covering and isolating the defect of 1) so as to eliminate its influence.
[0014]
These and other objects, advantages, and features of the invention are set forth in part in the description which follows, and will be apparent to one of ordinary skill in the art, upon review of the following, or appended claims. And can be realized and achievable as specifically indicated in FIG.
[0015]
(Detailed description of preferred embodiments)
For purposes of illustration, the present invention will primarily describe the formation of a thick GaN layer on a sapphire substrate using a suitable growth technique such as hydride vapor phase epitaxy (HVPE). It should be noted that the present invention uses other deposition techniques (Metal-Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), Sputtering, Vapor Deposition, etc.) to deposit GaN, AlN, InN and / or on substrates other than sapphire. The present invention can be applied to a case where materials containing these alloys are deposited and a case where these materials are arbitrarily combined. For illustrative purposes, the following embodiments are described with reference to a GaN substrate, but the invention is not limited to GaN substrates only. One skilled in the art will appreciate that the method is equally applicable to forming substrates of other III-nitrides and other III-V compounds.
[0016]
First embodiment
3 (a) to 3 (c) schematically show the growth of a thick, crack-free layer of a group III nitride such as GaN on sapphire according to a first embodiment of the present invention. I have. In FIG. 3 (a), a thick, defect-rich or rugged (uneven) first GaN layer 304 is grown on a substrate 301 using VPE or other related techniques (eg, HVPE, MOCVD, etc.). ing. The substrate 301 may be sapphire, a group III-V substrate, gallium arsenide, indium phosphide, gallium phosphide, zinc oxide, magnesium oxide, silicon, silicon oxide, silicon carbide, lithium aluminate, lithium gallate, and / or It may be composed of lithium aluminum gallate. In order to form the crystal facets 302, the GaN layer 304 may be grown, for example, by adjusting growth parameters or material composition during growth. Thereafter, as shown in FIG. 3B, a high-quality or extremely flat second GaN layer 305 is grown on the first GaN layer 304 by HVPE. After this second growth, the wafer is cooled to room temperature. Since the first GaN layer 304 relieves the residual stress between the second epitaxial GaN layer 305 and the substrate 301, cracks due to thermal mismatch do not occur during cooling. The first layer 304 and the second layer 305 form a heteroepitaxial layer 320. Layer 320 typically has a thickness greater than about 35 μm. The substrate 301 may be removed using polishing or other techniques, as shown in FIG.
[0017]
FIG. 4 schematically illustrates a series of steps included in a method for forming a thick layer on a thermally and / or lattice mismatched substrate according to the first embodiment of the present invention. In step 401, a prepared substrate is prepared. In step 402, process parameters for growing the first layer are set. For ease of reference, all process parameters and growth conditions are grouped together and [C ₁ ]. Process parameter [C ₁ ] Includes all relevant parameters relating to growth conditions for depositing the first layer, layer 302. [C ₁ ] Includes process conditions such as reactor pressure, reactor structure, reactor temperature, reactor temperature gradient, substrate temperature, gas flow rate, etc. used to deposit this thick layer. These variations may include, but are not limited to, for example, changes in substrate or reactor temperature, pressure, gas flow rates or gas flow rates, and reactor configurations. In step 403, a defect-rich or undulating first layer is grown on the prepared substrate. The thickness of this layer is preferably in the range from 10 to 300 microns. Condition [C ₁ ] Is set such that the deposited material has a high defect density or surface roughness, but remains essentially epitaxial in nature. Typical parameters [C ₁ Is usually as follows. The growth rate of layer 302 is typically about 50-200 microns / hr, preferably about 100 microns / hr. The temperature of the substrate 301 is usually about 1000 ° C. to about 1070 ° C., preferably about 1030 ° C. The ratio of the flow rate of the Group V material to the Group V material is designated herein as the V / III ratio and is usually from about 50 to about 250, preferably about 125. The pressure in the reactor is usually from about 0.9 to 1.5 atmospheres, preferably slightly above 1 atmosphere (1 atmosphere is approximately 14.7 lbs / in at sea level). ² Equivalent).
[0018]
In step 404, the growth condition is set to [C ₂ ] In terms of temperature, pressure, reactor structure, flow rate, etc. ₁ ]. Depending on the material used and the type of growth equipment, the set of parameters used in a reactor similar to that for depositing a defective or undulating layer such as layer 304 [C ₁ And a parameter [C used for depositing a low defect density or flat second layer. ₂ ]. In step 405, the growth condition [C ₂ ], A high-quality, low-defect or flat second layer such as the layer 305 is deposited on the first layer 302. Typical parameters [C ₂ Is usually as follows. The growth rate of layer 305 is typically about 5 to 50 microns / hr, preferably about 25 microns / hr. The substrate temperature is usually about 1000 ° C. to about 1070 ° C., preferably about 1030 ° C. The pressure in the reactor is usually from about 0.9 to 1.5 atm, preferably slightly above 1 atm. The V / III ratio is usually about 50 to about 250, preferably about 125.
[0019]
Since the second layer covers the first layer, the defects are buried and isolated, and do not propagate upward during growth. Alternatively, the second layer may be grown under conditions that promote lateral epitaxial growth. In this case, the growth is such that unevenness on the surface of the first layer is flattened and filled. Preferably, the thickness of this second layer is in the range of 10 to 300 microns. In step 406, the sample is cooled to room temperature without cracking the thick epitaxial layer or substrate, but the effect of the first layer minimizes residual stress. Since the film is grown under high-quality and high-stress growth conditions, the upper surface of the film has very high epitaxial properties. The deposited film has reduced the thermal stress sufficiently, but the thermal stress has been mitigated by defects and / or roughness of the underlying first layer. As a result, a thick layer of low-stress, crack-free material can be deposited on the thermally and / or lattice mismatched substrate, for example, depositing GaN on sapphire. In some cases, as shown in step 407 of FIG. 4, the sapphire substrate is removed after cooling, leaving, for example, the heteroepitaxial layer 320 as a separate GaN substrate.
[0020]
The steps described in the above embodiments can be modified without departing from the scope of the present invention. In an optional step, the growth rate can be kept constant, and the temperature of the substrate 301 can be raised from a state of growing an uneven film to a state of growing a flat film. Another option is a combination of two "extreme means", which slows down the growth rate and slightly raises the substrate temperature.
[0021]
In the following embodiments of the present invention, growing a defect-rich or undulating thick layer followed by a good or flat layer can be used to form a crack-free, high-quality thick epitaxial layer. This is a common process. The following embodiments focus on new steps or changes to the method used to achieve this common goal.
[0022]
Second embodiment
5 (a) to 5 (d) schematically show the growth of a thick, crack-free GaN layer on sapphire according to a second embodiment of the present invention. In FIG. 5A, a very thin buffer layer 506 made of, for example, GaN is grown on a substrate 501 by using a vapor phase epitaxy technique such as metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE). Can be. This substrate may have features in common with substrate 301 of FIG. 3 and may be constructed of similar types of materials. Usually, the thickness of the buffer is about 50 nm, and the buffer is grown at a lower temperature than during subsequent growth by HVPE. The buffer may be composed of AlN, InN, or an alloy of AlN, GaN and / or InN, ZnO, or MgO. Instead of a single buffer layer, multiple layers of different compositions can be used. In that case, the buffer layer may have a thickness exceeding 50 nm, or may include a plurality of layers grown at different temperatures by MOCVD. For example, after growing a buffer of one or more layers at a low temperature, one or more layers may be newly grown at a higher temperature, and in some cases further layers may be grown at a lower temperature. Included in the range. Buffers can be used to help reduce interfacial stresses caused by lattice and / or thermal mismatches, but also to help nucleate and grow the subsequent GaN layer. After the growth of the buffer, a second layer 507 of HVPE is deposited, which is thick, defective or rugged, as shown in FIG. Thereafter, as shown in FIG. 5C, a high-quality or extremely flat third GaN layer 508 made of HVPE is grown on the second GaN layer 507 made of HVPE. After this third growth, the wafer is cooled to room temperature. Since the second GaN layer 507 relieves the residual stress between the third epitaxial GaN layer 508 and the sapphire substrate 501, cracks due to thermal mismatch do not occur during cooling. As shown in FIG. 5D, the sapphire substrate 1 may be removed by polishing or other techniques. Then, layers 506, 507, and 508 form independent substrate 520.
[0023]
FIG. 6 schematically illustrates the sequence of steps involved in a method for forming a thick layer on a thermally and / or lattice mismatched substrate according to a second embodiment of the present invention. In step 601, a prepared substrate is prepared. In step 602, process parameters for growing the buffer layer (or layer) are set, and here [C ₁ ']. As described above, [C ₁ '] Includes all relevant parameters relating to the growth conditions for depositing the buffer layer (or layer). In the case of a buffer having a plurality of layers, a set of process conditions [C ₁ '] May be composed of a plurality of unique sub-sets, each sub-set being provided for each layer of the buffer structure. Typical parameters [C ₁ '] Is usually as follows. The thickness of the buffer layer 506 is generally about 5 to 100 nm, preferably about 30 nm. The growth rate of buffer layer 506 is typically about 0.1 to 1.0 microns / hr, preferably about 0.4 microns / hr. The substrate temperature is usually about 400 ° C. to about 700 ° C., preferably about 500 ° C. The pressure in the reactor is usually from about 0.9 to 1.5 atm, preferably slightly above 1 atm. The V / III ratio is usually about 4000 to about 7,000, preferably about 5500.
[0024]
In step 603, a buffer layer (or layer) is grown on the prepared substrate, and the total thickness is reduced to about 50 nm. In step 604, the growth condition is changed to [C ₂ ']. This change may involve the use of a different growth technique than that used in step 603, and in some cases, may require a new series of steps for unloading and loading the sample. May involve the use of equipment for the growth of Step 605 grows a defective or undulating second layer on the buffer layer. The thickness of this layer is preferably in the range from 10 to 300 microns. Condition [C ₂ '] Is set such that the deposited material has a high defect density or surface roughness, but remains essentially epitaxial in nature. Process conditions [C ₂ '] Is usually the set of conditions [C ₁ ] Is set within the same range as that set in [1]. In step 606, the growth condition is set to [C ₃ ] In terms of temperature, pressure, reactor structure, flow rate, etc. ₂ ']. In step 607, the growth condition [C ₃ , A good quality, low defect or flat third layer like the layer 508 is deposited on the second layer 506. Process conditions [C ₃ ] Is usually the set of conditions [C ₂ ] Is set within the same range as that set in [1]. Since the third layer covers the second layer, the defects are buried and isolated, and do not propagate upward during growth. In addition, the third layer may be grown under conditions that promote lateral epitaxial growth. In this case, the growth is such that unevenness on the surface of the second layer is flattened and filled. Preferably, the thickness of this third layer is in the range of 10 to 300 microns.
[0025]
In step 608, the sample is cooled to room temperature without cracking the thick epitaxial layer or substrate, but the effect of the second layer minimizes residual stress. Since the film is grown under high-quality and high-stress growth conditions, the upper surface of the film has very high epitaxial properties. The deposited film has reduced the thermal stress sufficiently, but the thermal stress has been mitigated by defects and / or roughness of the underlying second layer. As a result, a thick layer of low-stress, crack-free material can be deposited on the thermally and / or lattice mismatched substrate, for example, depositing GaN on sapphire. In some cases, as shown in step 609 of FIG. 6, after cooling, the sapphire substrate 501 is removed to obtain an independent GaN substrate, such as the substrate 520.
[0026]
Third embodiment
FIGS. 7 (a) to 7 (d) schematically show the growth of a thick, crack-free GaN layer on sapphire according to a third embodiment of the present invention. 7A, a buffer layer structure 702 is formed on a substrate 701 of the type described with reference to FIG. The buffer layer structure 702 typically includes a lower GaN layer 709 and a silicon dioxide (SiO 2) layer. ₂ ) And a layer 711 formed by epitaxial lateral growth (ELOG). Since the mask material suppresses the growth of GaN, the growth occurs only in the unmasked regions. This promotes lateral growth of the ELOG layer 711 and partially isolates the substrate from this layer and subsequent epitaxial layers. Such separation reduces stresses due to thermal and / or lattice mismatches remaining in the epitaxial layer and is achieved by reducing the effective contact area between the substrate and the epitaxial layer.
[0027]
Typically, these two GaN layers are grown using vapor phase epitaxy techniques such as MOCVD or HVPE to reduce the overall buffer structure thickness to about 1 micron, although other growth processes and buffer structure thicknesses are within the scope of the present invention. Contained within. Each layer of this structure may have a different composition, for example, layers 709 and 711 can each be composed of GaN, AlN, InN, or an alloy of these components. The patterned mask layer 710 is made of SiO ₂ , Silicon nitride (Si ₃ N ₄ ), Polysilicon, high melting point metal, or any combination of these components. These are given by way of example only and are not intended to limit the scope of embodiments of the present invention, so other suitable materials may be used to achieve a similar effect. Also, each layer may be composed of one set of sub-layers. To further reduce the level of residual stress or to promote lateral growth, layers 709, 710, and / or 711 may each comprise two or more layers of different compositions. Usually, the thickness of the lower layer 709 is about 1 micron, the thickness of the patterned mask layer 710 is about 500 nm, and the thickness of the ELOG layer 711 is about 1 to 4 microns. Other layer thicknesses useful for reducing residual stress or improving nucleation or lateral growth of layer 711 are also within the scope of embodiments of the present invention.
[0028]
After the growth of the buffer layer structure 702, a thick, defective or undulating HVPE layer 712 is deposited, as shown in FIG. 7 (b). Thereafter, as shown in FIG. 7C, a high-quality or extremely flat GaN layer 713 made of HVPE is grown on the third GaN layer 712 made of HVPE. After the growth of layer 713, the wafer is cooled to room temperature. Due to the following two complementary effects, there is no crack due to thermal mismatch during cooling. As described in the first embodiment of the present invention, the third GaN layer 712 reduces the residual stress between the fourth epitaxial GaN layer 713 and the sapphire substrate 701. In addition, the patterned mask layer 710 and the lateral growth layer 711 serve to mechanically separate the thick HVPE layers 712 and 713 from the sapphire substrate 701. As shown in FIG. 7D, the substrate 701 and a part of the buffer layer structure 702 can be removed by polishing or other techniques. Typically, the patterned layer and a portion of the lateral growth layer 711 are removed. The remaining portion of ELOG layer 711 and layers 712 and 713 form a separate III-nitride substrate 720.
[0029]
FIG. 8 schematically illustrates a series of steps included in a method for forming a thick layer on a thermally and / or lattice mismatched substrate according to a third embodiment of the present invention. In step 801, a prepared substrate is prepared. In step 802, process parameters for growing the first buffer layer (or layer) are set, here [C ₁ '']. A set of process parameters [C ₁ ''] Includes all relevant parameters relating to growth conditions for depositing a first buffer layer (or layer) such as layer 709. Process parameter [C ₁ ''] Is usually the same as that described with reference to FIG. 6 [C ₁ '] Is set within the same range as set in []. In step 803, a first buffer layer (or layer) is grown on the prepared substrate to a total thickness of about 1 micron. In step 804, the sample is cooled and removed from the reactor. In step 805, an appropriate mask material is deposited on the sample, and then patterned using an appropriate method such as lithography or etching. In step 806, the growth condition is set to [C for growing the ELOG layer (or layer) such as the ELOG layer 711. ₂ '']. Parameter set [C ₂ ''] Can include the step of reloading the sample into the reactor for growth. In step 807, an ELOG layer (or layer) is grown to a typical thickness of 1 to 10 microns. Parameter [C ₂ ''] Is usually as follows. When the ELOG layer 711 is grown at a high growth rate by HVPE, [C ₂ ″] Is the parameter [C ₁ ] May be included for undulating layers. When the ELOG layer 711 is grown by MOCVD, a set of parameters [C ₂ ''] Can be made as follows. The substrate temperature may be between about 970C and about 1070C. The pressure in the reactor is usually less than 1 atmosphere, preferably about 0.1 atmosphere. The growth rate of the ELOG layer 711 may be about 0.5 microns / hr to about 5 microns / hr, but is preferably about 2 microns / hr. The V / III ratio is usually about 500 to about 5000, preferably about 1500. Since the ELOG layer is grown on the mask layer through the underlying mask layer, the effective contact area between the epitaxial growth layer and the sapphire substrate is reduced. When the ELOG layer 711 reaches the upper surface of the patterned layer 710, it then expands laterally. Lateral growth typically prevents propagation of vertical defects from the sapphire-GaN interface. In the present invention, the second buffer layer also promotes cracking at the interface between the substrate 701 and the buffer layer structure 702.
[0030]
In step 808, the growth condition is set to [C ₃ '], But using a different growth technique than that used in step 806, and possibly another growth step which may require a new series of steps for loading and unloading the sample. May be used. Step 809 grows a highly defective or undulating third layer, such as layer 712, on the buffer layer. The thickness of this layer is preferably in the range from 10 to 300 microns. [C ₃ '] Is usually the parameter [C ₁ ] Is set within the same range as described above. Condition [C ₃ '] Is set such that the deposited material has a high defect density or surface roughness, but remains essentially epitaxial in nature.
[0031]
In step 810, the growth condition is changed to [C ₄ ] In terms of temperature, pressure, reactor structure, flow rate, etc. ₃ ']. In step 811, the growth condition [C ₄ ], A high-quality, low-defect or flat fourth layer such as the layer 713 is deposited on the third layer. [C ₄ ] Is usually the same as [C] in FIG. ₂ ] Is set within the same range as described above. Since the fourth layer covers the third layer, the defects are buried and isolated, and do not propagate upward during growth. In addition, the fourth layer may be grown under conditions that promote lateral epitaxial growth, and in this case, the growth is such that unevenness on the surface of the third layer is flattened and filled. Preferably, the thickness of this fourth layer is in the range of 10 to 300 microns. In step 812, the sample is cooled to room temperature without cracking the thick epitaxial layer or substrate. Residual stress is due to both the effect of the third layer as described in the first embodiment of the invention and the effect of the mask layer and the layer grown therethrough, which reduce the contact between the substrate and the thick epitaxial layer. Minimized. Since the film is grown under high-quality and high-stress growth conditions, the upper surface of the film has a very high epitaxial property and is safely isolated from the influence of the sapphire substrate. As a result, a thick layer of low stress, crack free material can be deposited on the thermal and / or lattice mismatched substrate, for example, depositing GaN on sapphire. In some cases, as shown in step 813 of FIG. 8, the sapphire substrate is removed after cooling to obtain an independent GaN substrate such as an independent substrate 720.
[0032]
Fourth embodiment
9 (a) to 9 (c) schematically show the growth of a thick, crack-free independent GaN substrate according to a fourth embodiment of the present invention. In FIG. 9A, a first GaN layer 904 having a large number of defects or many undulations is grown on a sapphire substrate 901 by HVPE. After the growth of this layer, the sample is cooled to room temperature and taken out of the reactor, and the sapphire substrate 901 is removed by polishing or other techniques as shown in FIG. The defect-rich or undulating GaN layer 904 can handle stress without cracking, so that cracks due to thermal mismatch do not occur during cooling. Thereafter, the now independent GaN layer is re-transported into the reactor, and as shown in FIG. 9C, a high-quality or extremely flat second GaN layer 905 is formed on the first GaN layer 904 by HVPE. Grow. Since this second layer 905 is homoepitaxially grown, it is free from the thermal and lattice mismatch constraints imposed by the sapphire substrate 901 and does not crack or undergo thermal stress upon cooling. As a result, a high-quality independent GaN substrate 920 is obtained.
[0033]
FIG. 10 schematically illustrates a series of steps included in a method for forming a thick, crack-free independent GaN substrate according to a fourth embodiment of the present invention. In step 1001, a prepared substrate is prepared. In step 1002, process parameters for growing a first III-V layer, such as layer 904, are set, here [C ₁ ''']. As already mentioned, [C ₁ '''] Includes all relevant parameters relating to the growth conditions for depositing the first layer. Step 1003 grows a defect-rich or undulating first layer on the prepared substrate. Preferably, the thickness of this layer is in the range of 20 to 300 microns. Condition [C ₁ '''] Is set so that the deposited material has a high defect density or rough surface, but remains essentially epitaxial in nature. [C ₁ '''] Is usually the parameter of [C ₁ ] Is set within the same range as described above. In step 1004, the sample is cooled to room temperature without cracking, but residual stress is minimized due to the effect of the defective or undulating first layer. In step 1005, the sapphire substrate is removed to obtain a rugged or defective, independent GaN layer. Although this independent layer is a single crystal, it is not suitable as a GaN substrate applied to a device due to its defect density and / or undulation. Step 1006 includes a series of steps for reloading the sample into the reactor [C ₂ ''']. In step 1007, the growth condition [C ₂ Based on “″], a second layer of good quality with few defects or flatness such as the layer 905 is deposited on the first layer. [C ₂ '''] Is usually the parameter of [C ₂ ] Is set within the same range as described above. Since the second layer covers the first layer, the defects are buried and isolated, and do not propagate upward during growth. Alternatively, the second layer may be grown under conditions that promote lateral epitaxial growth. In this case, the growth is such that unevenness on the surface of the first layer is flattened and filled. Preferably, the thickness of this second layer is in the range of 10 to 300 microns. In step 1008, the sample is cooled to room temperature without cracking, but there is no thermal mismatch since the sapphire substrate has been removed and not present in step 1005 to form a separate substrate 920.
[0034]
Fifth embodiment
FIGS. 11 (a) to 11 (d) schematically show the growth of a thick, crack-free independent GaN substrate according to a fifth embodiment of the present invention. In FIG. 11A, a thin GaN buffer layer 1106 can be grown on a substrate 1101 using a vapor phase epitaxy technique such as MOCVD or HVPE. Usually, the thickness of the buffer is about 50 nm, and the buffer is grown at a lower temperature than during subsequent growth by HVPE. The buffer may be made of AlN, InN, or an alloy of AlN, GaN and / or InN, ZnO, or MgO. Instead of a single buffer layer, a plurality of layers having different compositions can be used. In that case, the buffer layer may have a thickness of more than 50 nm or may be composed of a plurality of layers grown at different temperatures by MOCVD. For example, after growing a buffer composed of one or more layers at a low temperature, growing one or more new layers at a higher temperature and further growing the last layer at a low temperature are also within the scope of the present invention. Contained within. Buffers can be used to reduce interfacial stresses due to lattice and / or thermal mismatch, but may also be used to promote nucleation and growth of the subsequent GaN layer. After the growth of the buffer, a second layer 1107 of HVPE is deposited, which is thick, defective and rugged, as shown in FIG. 11 (b). After the growth of this layer, as shown in FIG. 11C, the sample is cooled to room temperature, taken out of the reactor, and the sapphire substrate 1101 is removed by polishing or other techniques. The second GaN layer 1107 having many defects or undulations can cope with stress without generating cracks, so that cracks due to thermal mismatch do not occur during cooling. Thereafter, the now independent GaN layer is re-loaded into the reactor, and a high-quality or extremely flat third GaN layer is formed on the second GaN layer 1107 by HVPE as shown in FIG. Grow 1108. Since this third layer 1108 is homoepitaxially grown, it is free from the thermal and lattice mismatch constraints imposed by the sapphire substrate 1101 and does not crack or undergo thermal stress upon cooling. . As a result, a high-quality independent GaN substrate 1120 is obtained.
[0035]
FIG. 12 schematically illustrates a series of steps included in a method for forming a thick, crack-free independent GaN substrate according to a fourth embodiment of the present invention. In step 1201, a prepared substrate is prepared. In step 1202, process parameters for growing the buffer layer (or layer) are set, and here [C ₁ '''']. Parameter set [C ₁ """] Includes all relevant parameters relating to the growth conditions for depositing the buffer layer (or layer). In the case of a buffer having a plurality of layers, a set of process conditions [C ₁ """] May be composed of a plurality of unique sub-sets, each sub-set being provided for each layer of the buffer structure. In step 1203, a buffer layer (or layer) such as the layer 1106 is grown on the prepared substrate, and the total thickness is reduced to about 50 nm. [C ₁ """] Is usually the parameter of [C ₁ '] Is set within the same range as described in regard to []. In step 1204, the growth condition is set to [C ₂ ''''). This change may involve the use of a different growth technique than that used in step 1203, and in some cases, may require a new set of steps for unloading and loading the sample. May involve the use of equipment for the growth of In step 1205, a second layer having a defect or undulation, such as the layer 1107, is grown on the buffer layer. Preferably, the thickness of this layer is in the range of 20 to 300 microns. [C ₂ """] Is usually the parameter [C ₁ ] Is set within the same range as described above. Condition [C ₂ """] Is set such that the deposited material has a high defect density or rough surface, but remains essentially epitaxial in nature. In step 1206, the sample is cooled to room temperature without cracking, but residual stress is minimized due to the effect of the defective or undulating second layer. In step 1207, the sapphire substrate is removed to obtain a rugged or defect-rich independent GaN layer. Although this independent layer is a single crystal, it is not suitable as a GaN substrate applied to a device due to its defect density and / or undulation. Step 1208 includes a series of steps to reload the sample into the reactor [C ₃ '']. In step 1209, the growth condition [C ₃ ''], Deposit a high quality, low defect or flat third layer, such as layer 1108, over the second layer. [C ₃ ″] Is usually the parameter of FIG. ₂ ] Is set within the same range as described above. Since the third layer covers the first layer, the defects are buried and isolated, and do not propagate upward during growth. In addition, the third layer may be grown under conditions that promote lateral epitaxial growth. In this case, the growth is such that unevenness on the surface of the second layer is flattened and filled. Preferably, the thickness of this third layer is in the range of 10 to 300 microns. In step 1210, the sample is cooled to room temperature without cracking, but there is no thermal mismatch because the sapphire substrate was removed and not present in step 1207 to form a separate substrate 1120.
[0036]
Sixth embodiment
FIGS. 13 (a) to 13 (d) schematically show the growth of a thick, crack-free, independent GaN substrate according to a sixth embodiment of the present invention. In FIG. 13A, the buffer layer structure has a lower GaN layer 1309 and silicon dioxide (SiO 2). ₂ ), And a patterned mask layer 1310 and an upper GaN layer 1311. Since the mask material suppresses the growth of GaN, growth occurs only in unmasked regions. Thereby, the lateral growth of the upper GaN layer 1311 is promoted, and the substrate is partially separated from this layer and the subsequent epitaxial layer. Such separation reduces stresses due to thermal and / or lattice mismatches remaining in the epitaxial layer and is achieved by reducing the effective contact area between the substrate and the epitaxial layer.
[0037]
Typically, these two GaN layers are grown using a vapor phase epitaxy technique such as MOCVD or HVPE to reduce the overall thickness of the buffer structure to about 1 micron, but other growth processes and buffer structure thicknesses of the invention are also contemplated. Included in the range. Each layer of the structure may have a different composition. For example, the layers 1309 and 1311 can be made of GaN, AlN, InN, or an alloy of these components, respectively, and the layer 1310 is made of SiO ₂ , Silicon nitride (Si ₃ N ₄ ), Polysilicon, high melting point metal, or any combination of these components. These are merely examples and are not intended to limit the scope of embodiments of the present invention, and other suitable materials may be used to achieve a similar effect. Also, each layer may be comprised of a set of sub-layers, with layers 1309, 1310, and / or 1311 having different compositions to further reduce the level of residual stress or to promote lateral growth. Each of them can be composed of two or more layers. Usually, the thickness of the lower GaN layer 1309 is about 1 μm, the thickness of the patterned mask layer 1310 is about 500 nm, and the thickness of the upper GaN layer 1311 is about 1 to 10 μm. Other layer thicknesses useful for reducing residual stress or improving nucleation or lateral growth of layer 1311 are also within the scope of embodiments of the present invention.
[0038]
After the growth of the buffer layer structure, as shown in FIG. 13B, a third layer 1312 of HVPE, which is thick, has many defects, or has many undulations, is deposited. After the growth of this layer, the sample is cooled to room temperature and taken out of the reactor, and the sapphire substrate 1 is removed by polishing or other techniques as shown in FIG. The following two complementary effects do not cause cracks due to thermal mismatch during cooling. As described in the fourth embodiment of the present invention, the third GaN layer 1312 handles the residual stress generated by the sapphire substrate 1. In addition, the patterned mask layer 1310 and the lateral growth layer 1311 serve to separate the thick HVPE layer 1312 from the substrate 1301. Thereafter, the now independent GaN layer is re-transported into the reactor, and as shown in FIG. 13D, a high-quality or extremely flat fourth GaN layer 1313 Grow. Since this fourth layer 1313 is homoepitaxially grown, it is free from the thermal and lattice mismatch constraints imposed by the sapphire substrate 1301 and does not crack or undergo thermal stress upon cooling. As a result, a high-quality independent GaN substrate 1320 is obtained.
[0039]
FIG. 14 schematically illustrates a series of steps included in a method for forming a thick, crack-free independent GaN substrate according to a sixth embodiment of the present invention. In step 1401, a prepared substrate is prepared. In step 1402, process parameters for growing the first buffer layer (or layer) are set, here [C ₁ ''''']. Parameter set [C ₁ '''''] Includes all relevant parameters relating to the growth conditions for depositing the buffer layer (or layer). [C ₁ ["""] Is usually the parameter [C ₁ '] Is set within the same range as described in regard to []. In step 1403, a buffer layer (or layer), such as layer 1309, is grown on the prepared substrate and the total thickness is reduced to about 1 micron. In step 1404, the sample is cooled and removed from the reactor. In step 1405, a suitable mask material is deposited on the sample and then patterned using a suitable method such as lithography or etching to form a patterned layer, such as layer 1310. In step 1406, [C] for growing a layer (or layer) by ELOG such as the layer 1311 is used. ₂ ''''']. Parameter set [C ₂ '''''] Can include re-loading the sample into the growth reactor. In step 1407, a layer (or layer) of ELOG is grown to a typical thickness of 1 to 10 microns. [C ₂ ["""] Is usually the parameter [C ₂ ''] Is set within the same range as that described in regard to ''). Since the ELOG layer is grown on the mask layer through the underlying mask layer, the effective contact area between the epitaxial growth layer and the sapphire substrate is reduced. In step 1408, the growth condition is changed to [C ₃ '''], But using a different growth technique than that used in step 1406, and possibly another set of steps for unloading and loading the sample. It may involve the use of growth equipment. Step 1409 grows a highly defective or undulating third layer, such as layer 1312, on the buffer layer. Preferably, the thickness of this layer is in the range of 20 to 300 microns. [C ₃ '''] Is usually the parameter of [C ₁ ] Is set within the same range as described above. Condition [C ₃ '''] Is set so that the deposited material has a high defect density or rough surface, but remains essentially epitaxial in nature. In step 1410, the sample is cooled to room temperature without cracking. Residual stress is minimized due to the effect of the defective or undulating second layer and the effect of separating the epitaxial layer from the substrate by growing the mask and the second layer on the buffer structure. Is done. In step 1411, the sapphire substrate is removed to obtain a rugged or defect-rich independent GaN layer. Although this independent layer is a single crystal, it is not suitable as a GaN substrate applied to a device due to its defect density and / or undulation. Step 1412 includes a series of steps to reload the sample into the reactor [C ₄ ] To the growth conditions. In step 1413, the growth condition [C ₄ ], A high-quality, low-defect or flat fourth layer is deposited on the third layer. Since the fourth layer covers the third layer, the defects are buried and isolated, and do not propagate upward during growth. In addition, the fourth layer may be grown under conditions that promote lateral epitaxial growth, and in this case, the growth is such that unevenness on the surface of the third layer is flattened and filled. Preferably, the thickness of this fourth layer is in the range of 10 to 300 microns. In step 1414, the sample is cooled to room temperature without cracking, but there is no thermal mismatch since the sapphire substrate was removed and not present in step 1411 to form a separate substrate 1320.
[0040]
The embodiments described above are examples and should not be construed as limiting the invention. The teachings of the present invention are applicable to other types of devices and methods. The description of the present invention is intended to be illustrative, and not to limit the scope of the appended claims. Also, changes and modifications to the method will be apparent to those skilled in the art.
[Brief description of the drawings]
FIG.
FIGS. 1 (a) and 1 (b) are cross-sectional views schematically illustrating the heteroepitaxial growth of thick GaN on conventional (prior art) sapphire.
FIG. 2
1 schematically illustrates a conventional process for heteroepitaxially growing thick GaN.
FIG. 3
3 (a) to 3 (c) are cross-sectional views schematically illustrating thick and / or independent heteroepitaxial growth of GaN according to the first embodiment of the present invention.
FIG. 4
1 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a first embodiment of the present invention.
FIG. 5
5 (a) to 5 (d) are cross-sectional views schematically illustrating heteroepitaxial growth of thick and / or independent GaN according to the second embodiment of the present invention.
FIG. 6
4 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a second embodiment of the present invention.
FIG. 7
7 (a) to 7 (d) are cross-sectional views schematically illustrating heteroepitaxial growth of thick and / or independent GaN according to the third embodiment of the present invention.
FIG. 8
4 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a third embodiment of the present invention.
FIG. 9
9 (a) to 9 (c) are cross-sectional views schematically illustrating heteroepitaxial growth of thick and / or independent GaN according to the fourth embodiment of the present invention.
FIG. 10
4 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a fourth embodiment of the present invention.
FIG. 11
11 (a) to 11 (d) are cross-sectional views schematically illustrating thick and / or independent heteroepitaxial growth of GaN according to the fifth embodiment of the present invention.
FIG.
5 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a fifth embodiment of the present invention.
FIG. 13
13 (a) to 13 (d) are cross-sectional views schematically illustrating heteroepitaxial growth of thick and / or independent GaN according to the sixth embodiment of the present invention.
FIG. 14
9 schematically illustrates a process for heteroepitaxially growing thick and / or independent GaN according to a sixth embodiment of the present invention.

Claims (28)

a) a step of preparing a substrate;
b) growing a defect-rich first epitaxial layer with reduced residual stress due to the defect density in the layer;
c) growing a second layer of good quality and low defect density on the first layer;
Forming a crack-free heteroepitaxial layer comprising: The first and second layers are grown in-situ, and one or more process conditions may be defined between the growth of the first layer and the growth of the second layer. 2. The method of claim 1, wherein the tuning optimizes the growth of the defect-rich first layer and the good quality second layer. The method of claim 1, wherein the substrate and the first layer have different lattice constants and / or different coefficients of thermal expansion. The material of the substrate is sapphire, III-V substrate, gallium arsenide, indium phosphide, gallium phosphide, zinc oxide, magnesium oxide, silicon, silicon oxide, silicon carbide, lithium aluminate, lithium gallate, and 2. The method of claim 1, wherein the method is selected from the group consisting of one or more of lithium aluminum gallate. The first layer is grown by vapor phase epitaxy (VPE), chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). The method of claim 1. The second layer is grown by vapor phase epitaxy (VPE), chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). The method of claim 1. The method of claim 1, wherein the defect-rich epitaxial layer is fabricated by adjusting its growth parameters and / or material composition to form crystalline facets. The method of claim 1, wherein the defect density of the defect-rich epitaxial layer is determined by selecting its material composition and / or growth parameters. The method of claim 1, wherein the first epitaxial layer having a large number of defects has a different composition from the second layer having a good quality. The method according to claim 1, wherein the first and / or the second layer contains GaN, AlN, InN, or an alloy thereof. The method of claim 1, wherein the thickness of the crack-free heteroepitaxial layer exceeds 35 μm. The method of claim 1, wherein the substrate is removed after growing the first epitaxial layer. 2. The method of claim 1, further comprising: d) growing a third layer on the substrate before growing the first layer. 14. The method of claim 13, wherein the thickness of the third layer is less than the thickness of the second or first layer. 14. The method of claim 13, wherein all or part of the third layer is grown at a lower temperature than the second and first epitaxial layers. 14. The method of claim 13, wherein growing at least two of said first, second and third layers is performed in situ. 17. The method of claim 16, wherein adjusting one or more growth parameters to optimize growth of one or more of the first, second, and third layers. 14. The method of claim 13, wherein the substrate and the epitaxial layer have different lattice constants and / or coefficients of thermal expansion. The substrate is sapphire, a group III-V substrate, gallium arsenide, indium phosphide, gallium phosphide, zinc oxide, magnesium oxide, silicon, silicon oxide, silicon carbide, lithium aluminate, lithium gallate, and / or 14. The method according to claim 13, comprising one or more materials selected from the group consisting of lithium aluminum gallate. 14. The method of claim 13, wherein the first layer is grown by vapor phase epitaxy (VPE), chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE). 14. The method according to claim 13, wherein the material of all or part of the third layer is GaN, AlN, InN, an alloy thereof, zinc oxide, or silicon carbide. The third layer includes two or more sub-layers, each of which contains GaN, AlN, InN, an alloy thereof, zinc oxide, or silicon carbide, and wherein the adjacent sub-layers have different lattice constants. 14. The method of claim 13, comprising: 14. The method of claim 13, wherein said third layer comprises two or more layers, one or more of said layers being patterned layers. The patterned layer comprises silicon oxide, silicon nitride, mixtures thereof, silicon, refractory metals, refractory metal oxides, refractory metal nitrides, or other suitable refractory materials. 23. The method according to 23. The patterned layer includes two or more sub-layers, at least one of which is silicon oxide, silicon nitride, mixtures thereof, silicon, refractory metals, oxides of refractory metals, 24. The method of claim 23, comprising a melting point metal nitride or other suitable high melting point material. 14. The method according to claim 13, wherein said substrate is removed after growing said first or second layer. The method of claim 1, wherein the first layer has an undulating surface. The method of claim 1, wherein the second layer has a flat surface.

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